PIXUS S YL VESTR1S. 1 65 



in the formation by division of many new cells. The 

 older cells on the axial side become gradually trans- 

 formed into the tracheides and those on the peripheral 

 side into the elements of the phloem. 



Replacing the tracheides of the xylem are the sieve 

 cells of the phloem. The radial walls of the larger cells 

 have on them clusters of small perforations which are 

 known as sieve plates or disks. These sieve plates are 

 homologous with the bordered pits on the tracheides of 

 the xylem." At a little distance from the cambium 

 they become covered with a homogeneous substance, 

 the so-called callus plate, which completely interferes 

 with the function of the sieve cells. Though this callus 

 plate is subsequently dissolved, the sieve cells never 

 regain their activity, the protoplasm having by this 

 time disappeared from them. " 



The cells with brown and crystalline contents are 

 the true phloem parenchyma. A single row of them 

 is formed each season, so that the age of the stem may 

 be determined by these, " as also by the growth rings 

 of the xylem. 



The general arrangement of the tissues of the bun- 

 dles is in contrast to that in the fern. The xylem and 

 phloem here lie side by side, whence the bundle is 

 known as collateral. " 



The rigidity of the leaves of the pine is due to the 

 thickening of the cells of the epidermis, together with 

 the development of the layer or layers of hypodermal 

 fibers. 



21 Strasburger, Das botanische Practicum, p. 143. 



2U Strasburger, op. cit., p. 147. 



83 Strasburger, op. cit., p. 146. 



94 Russow, Vergl. Untersuch., fide DeBary, Comp. Anat., p. 319. 



